The present invention relates to a shift actuator for a transmission for operating, in the direction of shift, a shift lever of a transmission mounted on a vehicle.
As the shift actuator for a transmission for operating a shift lever of the transmission in the direction of shift, there is generally used a fluid pressure cylinder by using a fluid pressure such as air pressure or hydraulic pressure as a source of operation. The shift actuator employing the fluid cylinder requires piping for connection to the source of fluid pressure, requires an electromagnetic change-over valve for changing over the flow passage of the operation fluid, and requires space for arranging the above components, resulting in an increase in weight of the device as a whole.
In recent years, there has been proposed an actuator of the type of an electric motor as a shift actuator for a transmission mounted on a vehicle which is provided with neither a source of compressed air nor a source of hydraulic pressure. The shift actuator constituted by the electric motor can be constituted in a compact size as a whole and in a reduced weight since it needs neither the piping for connection to the source of hydraulic pressure nor the electromagnetic change-over valve, unlike the actuators that use fluid pressure cylinders. The actuators using electric motors, however, require a speed reduction mechanism for obtaining a predetermined operation force. As the speed reduction mechanisms, there have been proposed the one using a ball-screw mechanism and the one using a gear mechanism. However, the actuators using the ball-screw mechanism and the gear mechanism are not necessarily satisfactory in regard to durability of the ball screw mechanism and of the gear mechanism and in regard to durability and the operation speed of the electric motors.
As a shift actuator for a transmission that has excellent durability and a high operation speed, therefore, the present applicant has proposed, in Japanese Patent Application No. 2001-013163, a shift actuator for a transmission, comprising an operation rod that engages with an operation member coupled to a shift lever of the transmission, a magnetic moving means arranged on the outer peripheral surface of the operation rod, a cylindrical fixed yoke surrounding the magnetic moving means, and a pair of coils arranged side by side in the axial direction inside the fixed yoke.
The shift actuator produces no driving force when the shifting mechanism is shifted to a shift stroke end, i.e., to a gear-engaging position. On the other hand, the shifting mechanism operated by the shift actuator receives a force in a direction in which the gear disengages, at the time when the transmission transmits the power. In order to prevent the gear from undesirably disengaging, therefore, the shifting mechanism must be limited from moving toward the neutral position side in a state where the shifting mechanism has been shifted to the shift stroke end, i.e., to the gear-engaging position and hence, produces no driving force. Therefore, the shifting mechanism in the transmission is provided with a detent mechanism for holding a state in which it has been shifted to the shift stroke end, i.e., to the gear-engaging position.
The detent mechanism usually has a constitution in which a detent ball pushed by a detent spring is brought into engagement with a dent formed in the shift rod that constitutes the shifting mechanism. To operate the shift rod shifted to the shift stroke end, i.e., to the gear-engaging position toward the neutral position side, the detent ball which has engaged with the dent formed in the shift rod must be caused to move along the tilted surface of the dent against the resilient force of the detent spring. Therefore, the shift actuator requires a large driving force.
If the shift actuator has a self-holding function for holding a state where it has been shifted to the shift stroke end, i.e., to the gear-engaging position, the shift rod needs no detent mechanism or the detent force can be made to small, making it possible to use a shift actuator having a small driving ability.
It is therefore an object of the present invention to provide a shift actuator having a self-holding function for holding a state in which the shifting mechanism has been shifted to the shift stroke end, i.e., to the gear-engaging position.
According to the present invention, to accomplish the above-mentioned object, there is provided a shift actuator for a transmission, comprising an operation rod that engages with an operation member coupled to the shift lever of the transmission, a magnetic moving means arranged on the outer peripheral surface of said operation rod, a cylindrical fixed yoke surrounding said magnetic moving means, and a pair of coils arranged side by side in the axial direction inside said fixed yoke, wherein magnetic members are arranged on both sides of said pair of coils.
The magnetic members are arranged in a bobbin on which the pair of coils are wound.